Not Applicable
1. Technical Field of the Invention
The invention relates to a method for measurement of an object by means of a feeler element used with a coordinate measuring instrument and extending from a feeler extension, where the feeler element is brought into contact with the object and its position is then determined. The invention further relates to a coordinate measuring instrument for measurement of structures of an object by means of a feeler used with a coordinate measuring instrument and comprising a feeler element and a feeler extension, a sensor for optical determination of the feeler element and/or at least one target directly assigned thereto, and an evaluation unit using, wherein the structures can be calculated from the position of the optical system relative to the coordinate system of the coordinate measuring instrument and from the position of the feeler element and/or of the target measured directly using the optical system.
2. Description of Related Art
For measurement of the structures of an object, coordinate measuring instruments with electromechanically operating feelers are used with which the structure position is determined indirectly, i.e. the position of the sensing element (ball) is transmitted via a feeler pin. The attendant deformations of the feeler pin in conjunction with the active friction forces lead to a falsification of the measurement results. Because of the strong force transmission, measurement forces also result that are typically in excess of 10 N. The geometric design of such feeler systems limits these to ball diameters greater than 0.3 mm.
The three-dimensional measurement of small structures in the range of a few tenths of a millimeter and the sensing of easily deformed test specimens is therefore problematic, if not impossible. As a result of the not completely known error influences due to deformation by the feeler pin and feeler element, and the unknown sensing forces due to stick-slip effects for example, measurement uncertainties occur that are typically in excess of 1 xcexcm.
It is known from WO 93/07443 to indirectly determine the structure of an object by means of optical sensors, where a rigid feeler has at least three targets, which are measured for determination of a coordination measurement point using an angle sensor.
Another possibility for optical measurement of the structures of a body is described in WO 88/07656 by an interferomter system. This system comprises a feeler with a rod-like feeler extension at the end of which a ball is arranged that is brought into contact with the body whose position is to be determined. The feeler extension extends from a plate-like holder that is adjustable in three dimensions relative to the object. Retroreflectors extend from the holders and are subjected to beams emitted by interferometers. The reflected beams are then measured by the interferometers in order to permit measurement of the optical axis between the interferometers and the retroreflectors for the determination of the position of the object.
The publication US-Z.: Quality, April 1998, p. 20 ff contains the proposal of measuring structures of an object by means of a feeler element by determining its position with an optical sensor. Here it is important that the feeler is sharply imaged.
It is known from the publication US-Z.: American Machinist, April 1994, p. 29-32, to use various measuring systems for the determination of the geometry of a workpiece. In this case it relates to the possibility, one the one hand, of measuring a surface with a video camera, and on the other hand, of performing a tactile measurement, these being treated as alternatives.
In US-Z.: Tooling and Production, October 1990. p 76-78, a feeler is used optionally for purely tactile, i.e. mechanical measurement and for optical measurement to determine structures. In this case, the feeler contacting the body must also be clearly optically imaged at all times.
A corresponding mechanical-feeler coordinate measuring instrument is shown for example in German Patent 43 27 250 A1. Here a visual check of the mechanically sensing process can be made with the aid of a monitor by observation of the feeler head using a video camera. This feeler head can if necessary be designed in the form of a so-called oscillating crystal feeler that is cushioned upon contact with the workpiece surface. The video camera therefore permits bracing and control on the monitor of the position of the feeler ball relative to the workpiece or to the hole therein which is being measured. The measurement proper is conducted electromechanically, so that the above drawbacks remain valid.
An optical observation of a feeler head in a coordinate measuring instrument is also shown in German Patent 35 02 388 A1.
To determine the precise position of the machine axes of a coordinate measuring instrument, at least six sensors are attached on a sleeve and/or to a measuring head in accordance with German Patent 43 12 579 A1, for enabling the distance from a reference surface to be determined. The sensing of the object geometries is not dealt with in detail here, instead a proximity-type process as a substitute for the classic incremental path measurement systems is described.
U.S. Pat. No. 4,972,597 describes a coordinate measuring instrument with one feeler, of which the feeler extension is pretensioned in its position by a spring. A feeler extension section passing inside the housing has two light-emitting elements located at a distance from one another for determining by means of a sensor element the position of the feeler extension, and hence indirectly that of a feeler element arranged on the outer end of the feeler extension. The optical system here also replaces the classic path measurement systems of electromechanical feeler systems. The sensing process proper is again achieved by force transmission from the feeler element to the feeler pin via spring elements to the sensor. The aforementioned problems with bending and sensing force remain here too. This method is indirect.
To measure large objects such as aircraft components, feeler pins with light sources or reflecting targets are known, the positions of which are optically measured (German Patents 36 29 689 A1, 26 05 772 A1, and 40 02 043 C2). The feelers themselves are moved manually or by using robotics along the surface of the body to be measured.
With this method, the position of the feeler element is stereoscopically determined in its position by triangulation or similar means. The resolution of the overall measurement system is hence directly limited by the sensor resolution. The use of such systems is therefore possible only in the case of relatively low requirements as regards the relationship of measurement area and accuracy. In practice their use is limited to the measurement of large parts.
Aiming at the position of the feeler element using a microscope is also known. In this case, the transmitted-light method is used, so that only structures such as all-through holes or similar can be measured in respect of their diameters. In view of the visual evaluation in the microscope and the separate arrangement of feeler element and optical observation system, neither measurement of more complex structures (distances in complex geometries, angles etc.) nor automatic measurement is possible. Systems of this type are as a result highly prone to faults and are therefore not offered on the market.
The problem underlying the present invention is to develop a method and a coordinate measuring instrument of the type mentioned at the outset such that any structures can be determined with a high degree of measurement accuracy, with the aim of precisely determining the position of the feeler element to be brought into contact with the object. In particular, it should be possible to measure out bores, holes, undercuts or similar, and to determine structures in the range between 50 and 100 xcexcm with a measuring accuracy of at least xc2x10.5 xcexcm.
The publication US-Z.: Plastics World, August 1989, No. 8, includes an illustration of a feeler element whose position is optically measured. As this illustration makes clear, a feeler is used that does not permit measurement of very small dimensions or of materials that are very soft and hence must not be subjected to high sensing forces, as otherwise a falsification of the geometry would result. An appropriate disclosure is also made in US-Z.: Quality, January, 1990, as the illustration makes clear.
A bore measuring microscope is known from the publication of Carl Zeiss Jena, Technische Messgerate, p. 54 and 55. In this teaching only the distance between two diametrically opposite points of a bore to be measured is determined under microscopic observation using the transmitted light method.
The problem is solved in accordance with the invention substantially in that the feeler element is connected via an elastic-to-bending shaft as the feeler extension to the coordinate measuring instrument, in that the position of the feeler element or of a target extending from the elastic-to-bending shaft and directly assigned to the feeler element is directly determined with an optical sensor. For measurement of the structure of the object using the optical sensor, certain coordinates of the feeler element or of the target are linked with those of the coordinate measuring instrument, with the position being determined in the transmitted light or reflected light method and/or by self emission of the feeler element or target. Here the feeler element and/or the at least one target is moved from the area of the optical sensor into the position to be measured. In other words, the feeler is moved towards the object from its side facing towards the sensor. The feeler and sensor are here adjustable as a unit inside a coordinate measuring instrument and their joint position can be measured with high precision. This is followed by a linked movement that ensures relatively low uncertainty in the results. Here the position in particular of the feeler element and/or of the at least one target is determined using the sensor by means of light beams reflecting from and/or penetrating the object and/or emanating from the feeler element or target.
In accordance with the invention, the position of the feeler element resulting from contact with the object is determined optically, in order to measure the shape of a structure directly from the position of the feeler element itself or of a target. Here the deflection of the feeler element can be measured by displacement of the image on a sensor field of an electronic image-processing system using an electronic camera. It is also possible to determine the deflection of the feeler element by evaluation of a contrast function of the image. A further possibility for ascertaining the deflection is to determine it from a size change of the target image, from which results the geometrical-optical correlation between object distance and enlargement. Also, the deflection of the feeler element can be determined by the apparent target size change resulting from the loss of contrast due to defocusing. As a general principle, the deflection vertical to the optical axis of the electronic camera is determined here. Alternatively, the position of the feeler element or of the at least one target assigned thereto can be determined by means of a photogrammetric system. If several targets are present, their position can be optically measured and then the position of the feeler element computed, as there is a clear and firm correlation between this and the targets.
In accordance with the invention, and in a divergence from the previous prior art, indirect measurement of the position of the feeler element or of the target assigned thereto takes place in order to determine the structure of an object. Here the feeler element and the target have a clear spatial correlation to the extent that a relative movement to one another does not take place, i.e. short spacings are maintained. It is immaterial here whether the feeler extension from which the target or feeler element extends is deformed during the measuring process, since the feeler element or the target is not indirectly measured, as in the prior art, but directly. With the method in accordance with the invention, holes, bores, depressions, undercuts or other structures with an extent in the range of at least 50-100 xcexcm can be measured with an accuracy of at least xc2x10.5 xcexcm. This enables three-dimensional measurements of very small structures to be performed, a requirement which has long been felt for example in medical technology for minimally invasive surgery, in microsensor systems, or in automotive engineering to the extent that injection nozzles, for example, are concerned, but which has not yet been satisfactorily solved. Thanks to the direct measurement of the feeler element position or of the target clearly assigned and not movable relative thereto, a direct mechanical/optical measuring method using a coordinate measuring instrument is provided that operates with high precision and does not lead to falsifications of measuring results even if the feeler extension becomes deformed during the measuring process.
A coordinate measuring instrument of the type mentioned at the outset is characterized in that the feeler extension is designed to be elastic to bending. The feeler element and/or the at least one target can here be designed self-radiating and/or as a reflector.
The feeler element and/or the target should preferably be designed as a body such as a ball or cylinder spatially emitting or reflecting a beam.
In accordance with the invention, the feeler element is connected to a feeler extension such as a shaft that is designed to be elastic to bending. This connection can be made by gluing, welding or by any other suitable type of fastening. The feeler element and/or the target can also be a section of the feeler extension itself. In particular, the feeler extension or the shaft is designed as or incorporates a light guide via which the necessary light is supplied to the feeler element or to the target. The shaft itself can be designed as a feeler at its end or can incorporate a feeler. In particular, the feeler element and/or the target should be interchangeably connected to the feeler extension such as a shaft.
In order to determine almost any structure, it is furthermore provided that the feeler extends from a holder adjustable in five degrees of freedom. The holder itself can in turn form a unit with the sensor or be connected to the sensor.
It is also possible for the feeler element and/or the target to be designed as or to incorporate a self-lighting electronic element such as an LED.
In accordance with the invention, a feeler system for coordinate measuring instruments is proposed that combines the advantages of optical and mechanical feeler systems, and which can be used in particular for the mechanical measurement of very small structures where conventional feeler systems can no longer be employed. However, simple attachment and changing of optical measuring instruments for mechanical measuring tasks is also possible as a result.
For example, it is provided that a feeler element or sensing element or a target assigned thereto can be determined in its position by a sensor such as an electronic camera once the former has been brought into mechanical contact with a workpiece. Since the position of either the feeler element itself or the target connected directly to the feeler element is measured, deformations of a shaft receiving the feeler have no effect on the measuring signal. In the measurement, it is not necessary for the elastic behavior of the shaft to be taken into consideration, and plastic deformations, hystereses and drift effects of the mechanical connection between the feeler element and the sensor cannot impair measurement accuracy. Deflections in the direction vertical to the sensor axis such as the camera axis can be determined directly by displacement of the image in a sensor field in particular of an electronic camera. The evaluation of the image can be performed with an image-processing system already installed in a coordinate measuring instrument. This provides a two-dimensionally operating feeler system which can be easily connected to an optical evaluation unit.
For sensing the deflection in the direction of the optical system axis such as a camera axis, there are several possibilities in accordance with the invention, for example:
1. The deflection of the feeler element in the direction of the sensor axis (camera axis)is measured by a focus system as already known in optical coordinate measurement technology for focusing on workpiece surfaces. Here the contrast function of the image is evaluated in the electronic camera.
2. The deflection of the feeler element in the direction of the sensor axis or camera axis is measured by the imaging size of a target being evaluated, e.g. in the case of a circular or annular target the change in the diameter. This effect is the result of the geometrical-optical imaging and can be selectively optimized by the design of the optical unit. In coordinate measurement technology, so-called telecentric lenses are frequently used and are intended to achieve a largely constant enlargement even in the event of deviation from the focal plane. This is achieved by moving the optical entry pupil into xe2x80x9cinfinityxe2x80x9d. For the evaluation as described above, an optimization the other way round would be useful: even a minor deviation from the focal plane should result in a clear change of the imaging scale. This is achieved by for example moving the optical entry pupil to the level of the focal point on the object side. If possible a high depth of field should be achieved to permit high-contrast imaging of the target over a relatively wide distance range. An ideal optical unit as regards its imaging properties for the application described above would be for example a pin camera. By the use of an annular target, size changes resulting from lack of focus can be minimized: it is not the mean ring diameter that changes due to lack of focus in the first approach, but only the ring width.
3. In a third option too, the size change of the target is evaluated, however this change results from the combination of geometrical-optical size change and the apparent enlargement by out-of-focus edges. In comparison with the evaluation of the lack of focus function, this method takes advantage of the fact that the actual size of the target is invariable.
In accordance with the invention, direct measurement of a feeler element position is used for determining the structures of objects. Generally speaking, many different physical principles are usable for the direct measurement. Since the measurement of the feeler element deflection in a very large measurement range in the spatial sense must be very precise, for example to permit continuous scanning operations, and to allow for a large excess stroke during object sensing (e.g. for safety reasons, but also to reduce the effort needed for precise positioning), a photogrammetric method can also be used. Two camera systems with axes oblique to one another can be used. In general the evaluation techniques known from industrial photogrammetry can be used.
With two cameras xe2x80x9clookingxe2x80x9d for example obliquely toward the longitudinal direction of the feeler element or to the ends facing said feeler element of a feeler extension such as a shaft, all measuring tasks can be performed in which the feeler element does not xe2x80x9cdisappearxe2x80x9d behind undercuts. The use of a redundant number of cameras (e.g. three) permits measurement of objects with steep contours too. For measurement in small bores, a camera can be used that is arranged such that it is xe2x80x9clookingxe2x80x9d onto the feeler element in the longitudinal direction of the feeler element or feeler extension. As a general principle, a single camera aligned with the longitudinal direction of the feeler extension such as shaft holding the feeler element is sufficient in the case of two-dimensional measurements (e.g. for measurements in bores).
For the use of the feeler in accordance with the invention, an actively light-emitting feeler element or other active target is not essential. Particularly high accuracies can be achieved with light-emitting feeler balls or other light-emitting targets on the feeler extension. The light from one light source is here supplied to the feeler element such as ball or to other targets of the feeler extension for example via a light guide fiber which can itself be the feeler shaft or feeler extension. The light too can be generated inside the shaft or in the targets if these contain LEDs, for example. The reason for these designs is that electronic image systems such as photogrammetric systems, in particular those for microscopically small structures, require a high light intensity. If this light is directly supplied to the feeler element in targeted form, the necessary light intensity can be reduced considerably, and hence also the thermal load on the object during the measurement. If a ball is used for the feeler element, the result is an ideally high-contrast and ideally circular image of the feeler ball from every direction viewed. This applies in particular in the use of a volume-dispersing ball. Errors from imaging of structures of the object itself are avoided, since the object itself is only brightly illuminated in the immediate vicinity of the feeler ball. Here however the feeler ball image resulting from reflections on the object in practice always appears less bright than the feeler ball itself As a result, errors can be corrected without difficulty. Externally illuminated targets do not necessary have these advantages. It is also possible to design the targets fluorescent, so that incoming and outgoing light is separated in terms of frequency, and hence the targets too can be more clearly isolated from their surroundings in the image. The same considerations apply for the feeler element itself.
To measure in small bores or on very steep structures too, when the feeler element cannot be measured itself or not measured by several cameras due to shading, the position, the orientation and the curvatures of the light guide fiber in the visible part-areas can be measured in accordance with the invention by sensors or by photogrammetry. From this the position of the feeler element can be calculated, e.g. by applying the fiber curvature in the form of a parabola with linear or square term. The measurement with different excess strokes (more or less positioned into the object) and then taking the mean of the feeler element position increases measuring precision. Both optical and photogrametric measurement of the fiber is facilitated by a steady light emission of the fiber, which can be improved by the use of volume-dispersing fiber material, the application of a diffusely emitting layer on the fiber surface or another suitable selection of fiber composition and fiber geometry (e.g. production using material with relatively low refractive index).
It is also possible in accordance with the invention to attach further illuminated balls or other targets on the light guide fiber, to measure the position of these targets by photogrammetry in particular, and to calculate the position of the feeler element accordingly. Balls are here relatively speaking ideal and clear targets that are not otherwise present on the fiber. A good light incidence into the balls is achieved by disrupting the light guidance properties of the shaft, for example by mounting the volume-dispersing balls with through-holes onto the shaft, i.e. the feeler extension, and gluing them there. The volume-dispersing balls can also be affixed to the side of the shaft, which also permits light incidence, provided that the shaft carries light up to its surface, i.e. does not have a sheath at the fastening point. A particularly high accuracy is achieved when the feeler element position is experimentally measured (calibrated) as a function of the fiber position and fiber curvature (zones of fibers at some distance from feeler element). Here too the measurement of targets attached along the fiber is possible instead of measurement of the fiber itself
Calibration can for example be achieved by sensing a ball from different directions and with different forces (more or less xe2x80x9cpositioned intoxe2x80x9d the object), or by a known relative positioning of the feeler system relative to the clamped feeler ball.
The separation of the feeler element, such as feeler ball, and targets additionally reduces the possibility of disruption of the feeler element position measurement by reflections of the targets on the object surface.
In accordance with the invention, several feelers can be in use consecutively; for example, various feeler elements or feeler pins can be rotated into view with a simple changing unit (e.g. turret with several feelers). It is also possible in accordance with the invention for several feeler elements to be in operation at the same time. The active feeler element or feeler pin can for example be identified by switching off the lights of the non-active feeler pins or by other coding means such as target size, light color, target position in feeler coordinate system, modulation of the light and/or using attached models. Feeler pin measurements as standard in classic coordinate measurement technology are no longer essential in the feelers in accordance with the invention, since the feeler ball position and the feeler ball diameter can be measured with often sufficient precision by photogrammetric means.
Measurement with small feeler elements often entails a large number of destroyed feeler pins (feeler element, feeler extension). With the system in accordance with the invention, the feeler pins are inexpensive and easy to replace. Expensive sensors and the movement axes are generally not damaged or altered by collisions, since the distance from the feeler element can be quite large. For example, the shaft length can be greater than the movement range of the system, so a collision is not possible. A large feeler or ball deflection relative to the shaft length is possible without difficulty. The result is a high inherent safety of the system and good scannability. Also, high sensing speeds are possible without damaging the object surface.
Photogrammetric systems, or other known optically operating sensor systems, permit mathematical alignment of the object before the actual start of measurement thanks to the image information from the lens. This permits accurate sensing of the object in the actual tactile measurement.
There are in this system two types of elastic influences that can lead to measurement deviations:
1. The resilience of the object itself (in large ranges); influences from this can be extrapolated to zero by measurement with at least two sensing forces; and,
2. The local resilience from Hertzian stress between ball and object surface; these effects can if required (i.e. for high-precision measurement or for resilient objects) be eliminated by a measurement with at least two different sensing forces and extrapolation to the fictitious sensing force xe2x80x9czeroxe2x80x9d.
The extrapolation to the force xe2x80x9czeroxe2x80x9d in the second ease is possible since the deformation according to Hertz is equal to a constant multiplied with the (sensing force)⅔
xe2x80x83D=Kxc3x97F⅔
where:
D is deformation at the point of contact between object and feeler ball;
F is force (or a quantity proportional to the sensing force); and
K is constant.
D1=Kxc3x97F1⅔
D2=Kxc3x97F2⅔
D1xe2x88x92D2=Kxc3x97(F1⅔xe2x88x92F2⅔)
From the above is derived the value of K when the difference (D1xe2x88x92D2) is known from the measurement and when F1 and F2 are known. It is now possible to calculate the deformations D1xe2x88x92D2 in relation to sensing with xe2x80x9czeroxe2x80x9d force. The force-proportional values are for example the movement distances calculated starting from the first object contact.
Alternatively, these can also be measured with force sensors. A force sensor for example can be the fiber itself if its curvature is photogrammetrically measured or on the basis of changes in the light reflected/diffused back internally to the light source or in the emitted light. It is best to perform the measurement with several sensing forces for all high-precision measuring tasks, since the effective radii in the contact point between the object and the feeler element can vary greatly due to local waviness and roughness features.
If the Hertzian and the linear resilience are of the same order of magnitude, sensing with at least three forces is necessary, and both the linear and the Hertzian resilience constant must be determined in order to extrapolate to the fictitious xe2x80x9czeroxe2x80x9d force.
If the divergences from the ideal spherical form in small balls used as feelers are not negligible, with diameters of less than 0.1 m, a direction-dependent correction of the sensing point coordinates may be necessary. To measure the correction values, two methods are possible:
1. measurement of the deviations of the feeler element from the spherical form, performed independently of the feeler system with special measuring instruments; and
2. measurement of the deviations of the feeler element from the spherical form, performed by measurement of a reference ball with the feeler system itself.
As a general principle, it is also possible to select a different geometry form for the feeler elements than that of a ball, e.g. a cylinder, which can represent the fiber itself or the rounded end of the fiber itself as the feeler extension.
Since the feeler element (e.g. a ball) is more or less completely imaged depending on the direction of observation, and since dirt too has a very disruptive effect, it is best to determine the position of the feeler element with so-called robust compensation algorithms. These include for example the minimization of the sum of deviation amounts (so-called L1 standard).
Correction methods set forth above are however only necessary in extreme cases, without the teachings in accordance with the invention being generally affected as a result.
Generally speaking, the illumination of the feeler element, the targets or the shaft can be not only from the inside through the shaft, but also from the outside using suitable illumination devices.
One variant that is possible here is for the feeler element or targets to be retro-reflectors (triple reflectors, cat""s eyes, reflecting balls) that are externally illuminated from the camera viewing angle.
The feeler in accordance with the invention is generally not itself restricted to certain sizes of the measurement objects and feeler element itself. It can be used for measurement of single-dimensional, two-dimensional or three-dimensional structures. In particular the feeler extension can be designed as a light guide and have a diameter of 20 xcexcm. The diameter of the feeler element such as feeler ball should then be preferably 50 xcexcm. In particular, it is provided that the diameter of the feeler element is about 1 to 3 times greater than that of the feeler extension.
To increase the fracture strength of the feeler extension when light guides are used, the latter can have a surface coating such as Teflon or another fracture-inhibiting substance. Sheathing can be applied by sputtering, for example.
The spatial position of the feeler element can be determined using a two-dimensional measuring system when the feeler element has at least three targets assigned to it, the images of which are evaluated for determining the spatial position of the feeler element.
The invention also permits a scanning method for determining workpiece geometries. In particular, the images to be evaluated can be generated by a position-sensitive surface sensor.
Compared with purely mechanically measuring feeler systems, the teachings in accordance with the invention have the following advantages, among others:
Elastic and plastic influences and creepage effects of the mechanical holder and the sensing shaft do not affect the measuring result.
Very low sensing forces ( greater than 1 N) can be attained.
No precision mechanics are necessary.
Very small feeler elements and shaft diameters can be used.
The positioning of the feeler system can be optimally monitored by the operator using the optical system.
The systems can be directly attached to the existing optical system of a coordinate measuring instrument and the image signal evaluated using an existing image processor.
Low equipment expenditure thanks to adaptation to existing optical coordinate measuring instruments.
Compared with purely optically measuring feeler systems, the advantages are as follows:
The actual mechanical quantities are measured. Surface properties such as color and reflection characteristics do not affect the measurement result;
Measurements can be made on three-dimensional structures not accessible for purely optical systems. For example, the diameter and the form divergence of a bore can be measured at different height sections.